The ferroelectric lead germanate (Pb5Ge3O11) and its isomorphous compounds are important because of their uses as pyroelectric and electro-optic devices. Comparison of inter-planar d-spacings of Pb5Ge3−xSixO11 (x=0, 0.3, 0.7 and 1.00) suggests that there is no change in basic structure of Pb5Ge3−xSixO11 when Si is substituted for Ge in small quantity (x&lt;1). The dielectric properties of the Si-substituted compounds have been studied as a function of temperature (30 to 200°C). The ferroelectric-paraelectric phase transition has been observed at 185°C. The Si doping causes (a) Curie point to shift towards low temperature, (b) peak value of the dielectric constant to decrease and (c) phase transition diffuse. The fast increase in dielectric constant of pure Pb5Ge3O11 with temperature (beyond transition temperature) may be attributed to the development of space charge polarization in the system.